1. Field of the Invention
This invention relates to an information recording and reproducing apparatus, and more particularly to an information recording and reproducing apparatus wherein a recording pulse signal is first converted into an optical pulse signal by intensity modulation and the optical pulse signal is optically recorded onto a rotating information recording medium in accordance with a mark length recording method to form a recorded mark train, and the recorded mark train is then optically reproduced.
2. Description of the Prior Art
Conventionally, in an optical disk apparatus wherein information is optically recorded onto a rotating information recording medium and thereafter the information is optically reproduced, recording of the information onto the information recording medium is performed for example by irradiating an intensity modulated beam of light upon the surface of the information recording medium to raise the temperature at an area upon which the beam of light of high power is irradiated so as to vary the shape or crystal condition of the information recording medium, or by employing an external magnetic field to change the direction of magnetization of the information recording medium, thereby forming recorded marks on the information recording medium.
Recording and reproduction of information-by a conventional information recording and reproducing apparatus is described in detail with reference to FIG. 1.
In order to record information onto an information recording medium using the mark length recording method, recording data 401 are converted into intensity modulated optical pulses 402 by means of a semiconductor laser drive circuit and a semiconductor laser, and optical pulses 402 are irradiated upon the surface of the information recording medium to form recorded mark train 403 on the surface of the information recording medium. However, from the thermal conductivity characteristic of the information recording medium, the rear half of a portion of the information recording medium at which optical pulse 402 is irradiated has a greater area at which the temperature rises high than the front half, and consequently, recorded marks 411,412 and 413 have a shape like a tear-drop.
When recorded mark train 403 formed in this manner is reproduced, the front edge of reproduced signal 404 has a gentle inclination than the rear edge, and reproduced pulses 405 obtained by pulsation of reproduced signal 404 have greater timing jitter at the front edge than at the rear edge.
Meanwhile, when the distance between two arbitrary successive recorded marks such as recorded marks 412 and 413 decreases, an edge shift phenomenon 414 occurs wherein the front edge of recorded mark 413 following recorded mark 412 is shifted forward from the position at which it should originally be formed, due to the accumulation or the transfer of heat produced when recording mark 412 is formed.
Thus, as means for reducing the influence of heat produced when recording the immediately prior recorded mark, thereby reducing the edge shift phenomenon, a recording method (hereinafter referred to as pulse train recording method) which uses recording pulses obtained by dividing a single recording pulse into multiple pulses by a reference clock signal for formation of recording pulses or some other suitable clock signal is adopted. In particular, according to the recording method, multi-divided pulses 406 are produced from recording data 401 and then modulated by intensity modulation so that they are converted into pulse train optical pulses 407 to be used for recording. In this instance, since multi-divided pulses are used, the energy is dispersed, resulting in reduction of the difference between the areas at the front edge portion and the rear edge portion.
However, even where the pulse train recording method is adopted, since a phenomenon still occurs whereby the area of the rear half of a recorded mark within which the temperature of the surface of the medium rises high is greater than that of the front half, recorded marks 415 to 417 have a tear-drop shape and reproduced signal 409 has a gentle inclination at the front edge than at the rear edge, and reproduced pulse 410 obtained by pulsation of reproduced signal 409 exhibits greater timing jitter at the front edge than at the rear edge.
Further, since the position of the top of a recording pulse is fixed, as the distance between two arbitrary successive recorded marks such as recorded marks 416 and 417 decreases, the front edge of rear recorded mark 417 is shifted forward from the position at which it should originally be formed, due to accumulation or transfer of heat produced when forming front recorded mark 416. In other words, even if the pulse train recording method is adopted, the edge shift phenomenon 418 cannot be eliminated.
Furthermore, in the pulse train recording method, since a recording pulse is divided into multiple recording pulses, when the peak power of the laser beam is equal to the peak power of a laser beam in the ordinary mark length recording method described above, the energy amount of the beam of light irradiated upon the surface of the information recording medium is small. Accordingly, in order to stably form recorded marks, the peak power of the laser beam of recording pulses must be set higher than the power used in method of ordinary mark length recording.